Seven hours. That was how long photojournalist Kevin Lim had to wait at Capella Singapore hotel to capture four minutes' worth of pictures of North Korean leader Kim Jong Un and United States President Donald Trump at this month's historic summit.

In a talk organised jointly by The Straits Times and the National Library Board (NLB) yesterday, Mr Lim spoke about his memorable experiences at the event, and what it takes to get the perfect shot, to a crowd of more than 250 people.

His photographs from the summit made their way to publications such as Time magazine, and were on display at the NLB building, where the talk was held.

Mr Lim, an executive photojournalist who has been with The Straits Times for nine years, was chosen as Singapore's only pool photographer for the summit.

Despite photographers from various nations taking their share of photos during the event, Mr Lim's iconic shot of the two leaders framed by the flags of their nations was the one that went viral.

During the session, he spoke about some of the trials he faced during his mission to get that one perfect shot. "I had to reach the designated venue at 4am, go through three layers of security - American, Singaporean and North Korean - and wait in holding rooms for many more hours before I could even get a glimpse of the two leaders."

Mr Lim said although the job took a trying seven hours, he had prior experience waiting at crime scenes and had anticipated the long wait.

During the 90-minute askST@NLB session, the photojournalist shared humorous anecdotes about his struggle with getting to the right position at the right time.

"It was a real squeeze," he said. "A photographer had his legs around my neck, another was elbowing me, I was sweating, my knees were aching from kneeling - and then I found out that we had to stay in that position for another 15 minutes."

The audience, who were given about 40 minutes to ask questions, quizzed Mr Lim on matters ranging from how he felt five minutes before the summit to whether he would consider switching to mobile-phone cameras to take pictures.

Mr Lim, in reply to the former question, said he had been focusing on keeping a clear mind and adjusting the settings of his camera.

He warned the audience about the perils of overthinking as a photojournalist and noted that it would cloud his judgment.

The talk was streamed live on the Rings.TV application.